Sensor and evaluation system, in particular for double sensors for determining positions and limit values

ABSTRACT

The invention relates to a sensor and evaluation system for capturing data at a measuring station ( 3 ), in particular for use in double sensors for determining end positions and limit values. The individual sensors ( 1, 1′, 2, 2′, 16, 16 ′) are operated as sensor pairs ( 1″, 2″, 16 ″) by a largely shared control circuit consisting of two connecting lines ( 5, 6 ) with an electric current or voltage of alternate polarity. A state of a sensor ( 1, 1′, 2, 2′, 16, 16 ′) is transmitted on the corresponding half-wave of the sensor current and evaluated by a switching amplifier ( 13 ) in relation to polarity and amplitude. Said sensor and evaluation system reduces the number of connecting lines required. In addition, interference from neighboring sensors is largely excluded, so that sensors can be mounted more closely together in a system. Said sensor and evaluation system is especially suited for equipment required to comply with the DIN 19234 (NAMUR) standard.

TECHNICAL AREA

The invention relates to a sensor and evaluation system for acquiringdata at a measuring point, especially for double sensors for detectingfinal positions and limit values, consisting of at least two sensorsthat are connected via a control circuit to a remotely located switchingamplifier with a current source or voltage source, whereby a connectionwire leads from the current source or voltage source to the sensor andanother connection wire leads from the sensor to an evaluation unit, andthe sensor signal of each sensor normally consists of a change in thecurrent flow as compared to the current flow in the quiescent state ofthe sensor and said sensor signal is transmitted via the control circuitto the switching amplifier in order to be evaluated there in anevaluation unit and to be output at an output stage in the form ofsignals (e.g. binary ZERO, ONE) as a function of the sensor and of thetime, according to the generic part of claim 1.

STATE OF THE ART

Sensors are normally connected by two connection cables or by a two-coreconnecting line to a switching amplifier which supplies the sensors withdirect current or direct voltage. The sensor signal consists in a changein the strength of the direct current flowing through the sensor to theevaluation unit as compared to the current flow in the quiescent stateof the sensor. For example, an oscillator that is integrated into thesensor vibrates when the current circuit is closed and, in the quiescentstate, i.e. without an object to be measured, it consumes a certaincurrent, which is measured as the quiescent signal. If an object to bemeasured is brought closer to the active surface of the sensor, then thecurrent consumption of the sensor changes, for example, as a result ofeddy-current losses in the object to be measured, and thus the currentflow in the evaluation unit also changes. The sensor signal is measuredand amplified in the evaluation unit, and an output stage—depending onthe signal level, e.g. when the value exceeds or falls below a certainswitching threshold—generates a switching signal, e.g. binary “zero” or“one”. The switching signal informs the user whether a certain state ofthe object to be measured is present, for instance, a certain spatialposition or a certain pressure.

For many applications, it has to be determined whether the state of theobject to be measured lies within certain tolerance limits. Therefore,in order to ascertain these two limit values, two sensors, i.e. a doublesensor consisting of a sensor pair, with four connection wires areneeded. This calls for quite complex wiring, which is disadvantageousfor many applications. In particular in the realm of chemistry, electriclines should be reduced to a minimum. For this purpose, it is known tocombine the two control circuits for double sensors on one side, therebyreducing the four connection wires to three. However, when work is donewith inherently safe control circuits corresponding to DIN 19234 NAMUR),this has the drawback that the resultant sum current has to be takeninto account in the verification of the inherent safety since, withNAMUR which provides that each of two sensors is a two-wire directcurrent sensor that operates at 8.2 V with switch points operatingbetween 1.2 to 2.1 mA systems, failures such as line breaks or shortcircuits are checked via the current in the interface between the sensorand the switching amplifier.

Moreover, for numerous applications, more than two states of the objectto be measured are to be detected with one system consisting of severalsensors. Here as well, two connection wires per sensor or, ifapplicable, three per sensor pair constitutes quite complicated wiringand there is a need for subsequent electronic devices that are capableof receiving the connection cables and processing the correspondingsignals. This is complex in terms of assembly and documentation, andentails potential sources of error.

Another disadvantage of the state of the art is that inductive sensorscannot be mounted at a location too close to a sensor system since theyinfluence each other if they are in operation at the same time, therebydistorting the measured result.

DE 40 33 053 C1 describes a measured value acquisition and transmissiondevice, consisting of a pick-up unit and a detection unit, which can becoupled together inductively for purposes of measured valuetransmission. The measured value acquisition and transmission device issupposed to prepare and transmit measured data in such a way that, firstof all, environmental influences are eliminated to the greatest extentpossible and secondly, the measured data can be transmittedinterference-free and contact-free, even over long transmissiondistances. The elimination of environmental influences, for example,high temperatures, is achieved in that the pick-up unit—in addition to ameasuring oscillator circuit—also comprises a reference circuit. Theoscillation frequency of the measuring oscillator circuit changesprimarily as a result of changes in the measured quantity to bemonitored, for example, the pressure, but it is also influenced anddistorted by environmental influences such as, for example, temperaturechanges. The reference circuit mimics the measuring oscillator circuit,but it is not capable of reacting to changes in the measured quantity.Its frequency changes only with the environmental influences. Therefore,interfering environmental influences should be eliminated by forming thequotient of the output signal of the measuring oscillator circuit andthe output signal of the reference circuit. The quotient is formed in anelectronic computing circuit.

The output signal of the electronic computing circuit is transmittedinductively from the pick-up unit to a detection unit. The computingcircuit supplies a quasi digital signal that can be transmittedinductively without being very interference-prone. The information onthe state of the object to be measured is contained in the frequency ofthis signal or in the number of counting pulses per unit of time.

TECHNICAL OBJECTIVE

The invention is based on the objective of creating a sensor andevaluation system of the type described above, in which theconfiguration of the individual sensors to form a system is simplifiedand the wiring effort between the sensors and the switching amplifier isreduced, and which allows an individual monitoring of the operatingstate of the individual sensor, a close spatial configuration ofadjacent sensors without influencing each other and a compact design ofthe switching amplifier.

DISCLOSURE OF THE INVENTION AND ITS ADVANTAGES

With a sensor and evaluation system for acquiring data at a measuringpoint, especially for double sensors for detecting final positions andlimit values, of the type described above, the achievement of theobjective is characterized by the following features: a) two sensors—asensor pair—are associated together with each of the two connectionwires to form the control circuit; b) an alternating current oralternating voltage interface is associated with each sensor pair, andsaid interface is located in the vicinity of the sensors and is capableof transmitting positive signals to and from one sensor as well asnegative signals to and from the other sensor of the sensor pair,whereby the two sensors of the sensor pair are connected togetherantiparallel and are always operated alternately with respect to eachother; c) on the basis of the polarity of the signals, the switchingamplifier establishes the association of the sensor signals of a sensorpair that are transmitted via the same connection wires to the sensor inquestion; d) the current source or voltage source is a current source orvoltage source with an alternating polarity.

In another advantageous embodiment of the invention, in order to formthe alternating current or alternating voltage interface of theconnection wires that establish the connection to the switchingamplifier, a short connection wire with a series-connected diodebranches off in the vicinity of each of the sensors, whereby each sensorof the sensor pair is connected to two connection wires—one of them witha diode—having opposite polarity, i.e. the sensors of a sensor pair areconnected together antiparallel.

The alternating current/voltage interface is set up with reverse-connectprotection diodes that are already integrated into the sensor. Here, thesensors can be electronic sensors or mechanical contacts.

The connection wire leading to the switching amplifier is followed by atleast one polarized comparator as an evaluation unit with a storagedevice and said comparator generates two static signals at the outputstage, corresponding to the state of the sensors of the sensor pair.

The levels of the alternating current and the sensors correspond to DIN19234 (NAMUR) and, for each sensor pair, the current is providedseparately to the sensors for an evaluation of the signal for linebreaks and line short circuits.

Moreover, the control circuit is galvanically separated from the currentsource or voltage source and from the evaluation unit and the subsequentelements.

The sensors are polarized analog transmitters and the evaluation unitsare polarized analog inputs or A/D transducers with storage behavior.

With the sensor and evaluation system according to the invention, in anadvantageous manner, the connection wires necessary to connect thesensors to the switching amplifier are reduced from four, optionallythree, wires per sensor pair to two per sensor pair. As a result, thewiring effort is reduced and the necessary connection points on theswitching amplifier are reduced, as a result of which the latter can bedesigned more compactly.

With the sensor and evaluation system according to the invention, onesensor pair is actuated by a pair of wires with current having apolarity that alternates over time instead of with direct current.Current having a polarity that alternates over time is also designatedas alternating current below, whereby alternating current in the mostgeneral sense of the term is to be understood. An alternatingcurrent/alternating voltage interface ensures that the individual sensoris supplied with current or voltage having a fixed polarity and that thetwo sensors of a sensor pair always operate alternately. One of thesensors of a sensor pair is actuated, for example, with the positive ornegative half-wave of the alternating current, and the appertainingsensor signal consists of the amplitude of the positive or negativehalf-wave of the current transmitted to the evaluation unit, i.e. theinformation on the state of the individual sensor is contained in theamplitude of the particular half-wave of the signal transmitted by theshared control circuit. The evaluation unit associated with eachindividual sensor is only sensitive to signals of the specific polarity;signals of the opposite polarity are ignored or are evaluated by theevaluation unit that is associated with the other sensor of the sensorpair.

The evaluation unit can be a polarized comparator or a polarized analoginput or an A/D transducer. By dispensing with a continuous directcurrent sensor signal, two pulsed signals can be transmitted via thesame pair of lines by feeding in alternating current during thealternating operation of the sensors of a sensor pair. A stationarysignal at the output stage is achieved in that the evaluation unit isfollowed by a storage device, for example, a peak-value hold device, alow-pass filter or a sample-and-hold unit.

In addition to the possibility of a more compact design of the switchingamplifier, the cable savings also has the advantage that the entiresensor system can be wired with less effort and is thus easier tomaintain and less error-prone.

The wiring according to the invention of two or more sensors to form asensor system with reduced cabling can also be used when there are twosensors, one of which is used predominantly while the other is only usedoccasionally. Then, as the alternating current, a signal is used inwhich the positive and negative half-waves are of different lengths andwhich is optionally also switched manually.

The alternating current or alternating voltage interface in the area ofthe sensors can be formed simply in that, in the vicinity of each of thesensors, a short connection wire with a series-connected diode branchesoff from the connection wires that establish the connection to theswitching amplifier, and then each sensor of the sensor pair isconnected to two connection wires—one of them with a diode—havingopposite polarity.

In many sensors, especially NAMUR sensors, diodes are already integratedas reverse-connect protection diodes, so that only a fixed inputpolarity is permissible. In this case, external diodes can be dispensedwith when the alternating current interface is set up, and only onebranching of the connection wires in the input area of the sensors isnecessary in each case, whereby attention has to be paid to theantiparallel switching of the sensors.

The sensor and evaluation system according to the invention can be setup with any kind of sensors, mechanical or electric, analog or digital.The evaluation unit is adapted to the type of sensor (analog/digitaloperation). The sensor system according to the invention is especiallyadvantageous for installations where inherent safety, e.g. according toDIN 19234 (NAMUR), is required such as, for example, level monitoring oftanks containing explosive liquids or pressure measurement or thedetection of a valve state within such an installation. When theoperating state of the individual sensor is monitored, for instance,checking for line breaks, the current flow through the appertainingcontrol circuit is measured, whereby this flow must not fall below acertain limit.

In an advantageous manner, with the connection of the sensors accordingto the invention, the monitoring of the operating state can be carriedout sensor-selectively since, with the positive or negative half-wave ofthe alternating current signal, there are practically two independentsignals or pieces of information available.

In order to increase the operating safety of the sensor and evaluationsystem, it is advantageous for the control circuits to be galvanicallyseparated from the current source or voltage source and from theevaluation units and subsequent elements, for example, with atransformer that, at the same time, transforms the mains voltage down tolower voltages in the range of a few volts, as required by NAMUR.

Since the sensors in the sensor and evaluation system according to theinvention function at alternating times as a result of their operationwith alternating current, there is less mutual influencing ofelectronic, e.g. inductive, sensors that are located close to each otherthan is the case with simultaneous continuous operation with directcurrent. If there are more than two sensors, the sensors can beconnected together about twice as closely to form a sensor system thanis the case with operation with direct current since, as a rule, onlynon-adjacent sensors are operated at the same time and could influenceor interfere with each other; however, the individual sensor pairs arenot at less distance with respect to each other than in the case ofoperation with direct current.

In principle, it is also possible for more than two sensors to beoperated via a shared control circuit if there are synchronized samplingunits before the sensor and in the switching amplifier, so that onesensor is switched on briefly at a given point in time but never at thesame time as another sensor, and the analyzed sensor signal isassociated with the appropriate output stage. Wiring just two sensors,however, is much less complicated since actuating the sensors andassociating the signals can be achieved simply by means of the polarity,and the diodes needed to form the alternating current interface areoften already integrated into the sensors.

Short description of the drawings in which the following is shown:

FIG. 1 a block diagram of a sensor and evaluation system with twosensors (for example, one double sensor)

FIG. 2 a block diagram of part of a sensor and evaluation system withtwo sensors with an alternative structure of the alternating currentinterface

FIG. 3 a block diagram of a part of a sensor and evaluation system withtwo sensors with reverse-connect protection diodes

FIG. 4 a diagram for the representation of the sensor signals and thesignals at the output of the switching amplifier as a function of thestate of the sensors for one sensor pair.

FIG. 5 a block diagram of a sensor and evaluation system with more thentwo sensors.

PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a block diagram of a sensor and evaluation system with twosensors 1 and 1′ coupled to form a sensor pair 1″, or a double sensor1″. The sensors 1, 1′ serve for purposes of measured value acquisitionat a measuring point 3, e.g. for detecting two end positions of anobject to be measured that define a tolerance range, or for detecting apressure or a valve position. The sensor pair is connected by twoconnection wires 5, 6 to a switching amplifier 13, which contains asource of alternating current 9 for supplying current to the sensor pair1″ as well as evaluation units 10, 10′ and storage units 11, 11′ andoutput stages 12, 12′ for the evaluation and output of the sensor signalor of the information about the state of the sensor. The switchingamplifier is, for example, a buffer amplifier.

For the most part, the two connection wires 5 and 6 are associated withthe sensors 1 and 1′ of the sensor pair 1″ and form a largely sharedcontrol circuit 18. The connection of the sensor pair 1″ with theswitching amplifier 13 is thus achieved with just two connection wires5, 6, which can be very long, since the sensor 1, 1′ is generallylocated far away from the switching amplifier 13. All of the otherelectronic connections are short lines.

Since sensors are generally operated with direct current or directvoltage, in the vicinity of the sensors, there is an alternatingcurrent/voltage interface 7 for rectifying the alternating current orthe alternating voltage and said interface transmits the positivehalf-wave of the alternating current/voltage from and to sensor 1 orelse the negative half-wave of the alternating current/voltage from andto sensor 1′. The alternating current/voltage interface is formed inthat short connection wires 5′ and 6′ branch off from the connectionwires 5 and 6, whereby the sensor 1 is connected to the wires 5 and 6,while the sensor 1′ is connected to the wires 5′ and 6′, and there is aseries-connected diode 8′ or 8 between the branching site and the sensorinput at each of the wires 5 and 6′. The polarity of the wires and theswitching of the diodes (blocking/conducting direction) is selected insuch a way that the two sensors 1, 1′ of the sensor pair are connectedantiparallel, they react to signals of fixed, different polarity andthus, when alternating current is fed in, they are operated alternately,i.e. pulsed with the alternating current frequency.

An example of the signal course is shown in FIG. 1 and described in thetext below. The sensor 1 is actuated by the control circuit 18consisting of the connection wires 5 and 6, and the sensor 2 is actuatedby the control circuit 18 consisting of the connection wires 5 and 6,which are continued after the branching, and by the connection wires 5′and 6′. Via the connection wire 6, the alternating current signalgenerated by the current source 9 and modified by the sensors 1, 1′,depending on the state of the object to be measured in terms of thepositive and/or negative amplitude, is transmitted to the evaluationunits 10, 10′, whereby the evaluation unit 10 is associated with thesensor 1 and is sensitive to positive signals, while the evaluation unit10′ is associated with the sensor 1′ and is sensitive to negativesignals. The evaluation unit then generates a signal if the positive ornegative amplitude of the sensor signal lies above or below anadjustable threshold. A polarized comparator or a polarized analog inputor an A/D transducer, for example, can be used as the evaluation unit.It is also possible to use a shared evaluation unit per sensor pair, ifthis unit is able to separate positive and negative signals from eachother and it generates two output signals at the output stage which arethen associated with the individual sensors.

Since the sensor signal of the individual sensor and thus also thesignal generated by the evaluation unit 10, 10′ is pulsed, but since astationary signal is desired at the output stage 12, 12′, the evaluationunit and the output stage have storage elements 11, 11′ between them,for example, a peak-value hold device, a low-pass filter or asample-and-hold unit. Thus, a signal, e.g. valve open or closed, can begenerated at the output stage that is stationary on the time scale ofthe alternating current, but that can depict changes on the time scaleof the processes that occur at the measuring site.

With the schematic set-up shown, however, it is also possible to operatepredominantly one of the sensors of the sensor pair with direct currentand only to occasionally change the polarity of the current and thus toactuate and select the second sensor. This should also be understood inthe broad sense as referring to the operation of the sensor andevaluation system with current having an alternating polarity.

It is easy to form a sensor and evaluation system with a plurality ofsensors (19, 19′) by always connecting the individual sensors togetherpairwise as shown in FIG. 5. This makes it possible to set up a sensorsystem in which a mutual influencing of immediate neighbors can belargely ruled out by synchronized sampling units (20, 20′, 21, 21′)since directly adjacent sensors are not operated at the same time andthus, for instance, any inductive coupling remains without consequences.

FIG. 2 shows a block diagram of part of a sensor and evaluation systemconsisting of two sensors 16, 16′ that can also be a double sensor. Thelevels of the alternating current and the sensors (1, 1′, 2, 2′, 16,16′) can correspond to DIN 19234 (NAMUR). The current can be providedseparately to the sensors (1, 1′, 2, 2′) for each sensor pair (1″, 2″16″), for an evaluation of the signal for line breaks and line shortcircuits. The sensors (1, 1′, 2, 2′, 16, 16′) can be polarized analogtransmitters and the evaluation units (10, 10′) can be polarized analoginputs or A/D transducers with storage behavior. The alternating currentinterface 7 is set up in such a way that it is preceded by two diodes17, 17′ on inputs of the sensors 16, 16′ that correspond to each other,for example, the signal input, and antiparallel coupling to the longconnection wires 5, 6 with short connection wires 5′, 6′. The two diodescan already be integrated into an eight-pole component so that the usermerely has to couple the connection wires and the sensors with thecomponent in such a way that the sensors are connected antiparallel. Thecontinuation of the sensor and evaluation system along the longconnection wires 5, 6 corresponds to the set-up shown in FIG. 1.

FIG. 3 shows a block diagram of a part of a sensor and evaluation systemwith a sensor pair 2″ or with a double sensor consisting of two sensors2, 2′, whereby the sensors 2, 2′ are fitted with reverse-connectprotection diodes 14, 14′ according to DIN 19234 (NAMUR). Since thediodes needed to set up an alternating current interface 7 are alreadyintegrated into the sensors 2, 2′ as reverse-connect protection diodes14, 14′, here at the plus input, the antiparallel actuation of thesensors 2, 2′ can be effectuated with current having a fixed polarity inthat the plus input of sensor 2 is connected in parallel with the minusinput of sensor 2′ by means of connection wires 5, 5′, and the minusinput of sensor 2 is connected in parallel with the plus input of sensor2′ by means of connection wires 6, 6′ without the involvement ofadditional diodes. The connection of the sensor pair 2″ with theswitching amplifier, which is not shown here, is established by two longconnection wires 5, 6. The short connection wires 5′, 6′ serve only forthe parallel connection of the sensors 2, 2′.

A transformer 15 galvanically separates the control circuit 18(connection wires 5, 5′, 6, 6′) and the sensors 2, 2′ from the powersupply and the evaluation units as well as the subsequent elements. Thetransformer transforms the mains voltage to voltages in the range of afew volts so that the current flows—at a few mA—in the control circuit18 lie within the limits permitted by NAMUR.

FIG. 4 shows a diagram for the representation of the sensor signals andthe signals at the output of the switching amplifier as a function ofthe state of the sensors A, B of a sensor pair connected according tothe invention, e.g. sensors 2, 2′ shown in FIG. 2. In the top line, thevoltage present at the switching amplifier is shown as a function of thetime t. This is square-wave voltage that brings about a current flowwith a square course over time in the control circuit 18.

In the first column, the states of sensors A, B or of the sensor pairare shown schematically, whereby a bar in front of the sensor isintended to indicate that the switching threshold of the sensor has beenexceeded and the object to be measured is in a previously defined statethat is to be ascertained.

The second column shows the sensor current, the current through thecontrol circuit, as a function of the time with the same time axis asthe voltage shown in the first line.

In the last column, the output of the switching amplifier is shown inthe form of binary signals, zero and one, or as an equation that is notto be understood in the strictly mathematical sense. The numeral onemeans that the sensor is in the quiescent state, whereas a zero meansthat another, previously defined state has occurred (indicated by thebar in front of the sensor). The information A or B relates to thespecific sensor A or B and thus to the state whose occurrence is to bemonitored with the sensor A or B.

The third and fourth lines show how the signals of two sensors A or Bbehave in the connection according to the invention at an alternatingcurrent interface, but without the connection of the other sensor of thesensor pair in the quiescent state (no bar). Due to the alternatingvoltage, only the positive or negative current component can flowthrough sensor A or B (3^(rd) or 4^(th) line) as sensor current. In thequiescent state of the sensor, the current strength has an amplitude Îor −Î. The signal at the output of the switching amplifier is one inboth cases, corresponding to the quiescent state of the sensor or the“normal state” of the object to be measured.

The fifth and sixth lines show how the signals of two sensors A and Bbehave in the connection according to the invention at an alternatingcurrent interface, but without the connection of the other sensor of thesensor pair in the previously defined state of the object to be measured(bar) at which the sensor switches. Due to the alternating voltage, onlythe positive or negative current component can flow through sensor A orB (5^(th) or 6^(th) line) as sensor current. In the switched state ofthe sensor, the current strength has an amplitude Î or −Î, which is lessthan the amount of the amplitude Î in the quiescent state. If the amountof the amplitude of the sensor current lies below a threshold, then zerois signaled at the output. This is the case in the 5^(th) and 6^(th)lines, so that the signal at the output of the switching amplifier iszero in both cases, which indicates the previously defined state of theobject to be measured.

The seventh to tenth lines show how the signals of two sensors A and Bbehave in the connection according to the invention at an alternatingcurrent interface and combination to form a sensor system as shown inFIGS. 1 or 2.

In the 7^(th) line, both sensors A and B are in the quiescent state (nobar), the amount of the positive as well as of the negative amplitude isÎ. The time course of the sensor current follows the voltage that ispresent at the switching amplifier. Since the positive and negativehalf-waves are each evaluated on their own and since their amplitudelies above the switching threshold, the stationary signal one isgenerated simultaneously in each case at the output for the sensor A andthe sensor B.

In the 8^(th) line, both sensors A and B are in the switching state(with bar), the amount of the positive as well as of the negativeamplitude is I. In this case, the time course of the sensor current alsofollows the voltage that is present at the switching amplifier, but witha reduced amplitude I, which lies below the switching threshold. Thestationary signal zero is generated simultaneously in each case at theoutput for the sensor A and the sensor B.

In the 9^(th) line, the sensors A are in the switched state (with bar)and the sensor B is in the quiescent state (no bar). The positive andnegative half-waves of the sensor current have different amplitudes Î or−Î. Thus, the amount of the amplitude of the sensor current lies belowthe switching threshold for sensor A and above the switching thresholdfor sensor B. At the output, the stationary signal zero is generated forthe sensor A and, at the same time, the stationary signal one isgenerated for the sensor B. The situation in the 10^(th) line resultsfrom the situation described here by reversing the sensors A and B aswell as the mathematical signs of the sensor signals.

All in all, it is evident that two sensors of a sensor pair can alwaysbe operated alternately to each other by feeding in alternating currentthrough a shared control circuit and nevertheless, the same informationabout the state of the sensor and thus the state of the object to bemeasured can be transmitted with the same shared control circuit.

Commercial Utilization

The subject matter of the invention is suitable especially for use ininstallations where inherent safety, e.g. according to DIN 19234(NAMUR), is required such as, for example, level monitoring of tankscontaining explosive liquids or pressure measurement or the detection ofa valve state within such an installation. When the operating state ofthe individual sensor is monitored, for instance, checking for linebreaks, the current flow through the appertaining control circuit ismeasured, whereby this flow must not fall below a certain limit. Theusefulness of the invention consists especially in the fact that thesensor and evaluation system according to the invention can be set upwith any kind of sensors, mechanical or electric, analog or digital. Theevaluation unit is adapted to the type of sensor (analog/digitaloperation). Advantageously, the monitoring of the operating state can becarried out sensor-selectively since, with the positive and negativehalf-wave of the alternating current signal, there are practically twoindependent signals or pieces of information available.

List of the reference numerals: 1,1′,2,2′,16,16′,19,19′ sensor 1″,2″,16″sensor pair 3 measuring point 5,5′,6,6′ connection wire 7,7′ alternatingcurrent or alternating voltage interface 8,8′,14,14′,17,17′ diode(14,14′ reverse-connect protection diode) 9 current source or voltagesource 10,10′ evaluation unit 11,11′ storage element 12,12′ output stage13 switching amplifier 15 transformer 18 control circuit 20,20′,21,21′synchronized sampling unit

What is claimed is:
 1. Sensor and evaluation system for acquiring dataat a measuring point (3), especially for double sensors for detectingfinal positions and limit values, consisting of at least two sensors(1,1′,2,2′,16,16′) that are connected via a control circuit to aremotely located switching amplifier (13) with a current source orvoltage source (9), whereby a connection wire (5,5′) leads from thecurrent source or voltage source to the sensor (1,1′,2,2′,16,16′) andanother connection wire (6,6′) leads from the sensor (1,1′,2,2′,16,16′)to an evaluation unit (10,10′), and the sensor signal of each sensor(1,1′,2,2′,16,16′) normally consists of a change in the current flow ascompared to the current flow in the quiescent state of the sensor(1,1′,2,2′,16,16′) and said sensor signal is transmitted via the controlcircuit to the switching amplifier (13) in order to be evaluated therein an evaluation unit (10,10′) and to be output at an output stage(12,12′) in the form of signals (e.g. binary ZERO, ONE) as a function ofthe sensor (1,1′,2,2′,16,16′) and of the time, characterized by thefollowing features: a) two sensors (1,1′,2,2′,16,16′)—a sensor pair(1″,2″16″)—are associated together with each of the two connection wires(5,5′,6,6′) to form the control circuit; b) an alternating current oralternating voltage interface (7,7′) is associated with each sensor pair(1″,2″16″), and said interface (7,7′) is located in the vicinity of thesensors (1,1′,2,2′,16,16′) and is capable of transmitting positivesignals to and from one sensor (1,2,16) as well as negative signals toand from the other sensor (1′,2′,16′) of the sensor pair (1″,2″16″),whereby the two sensors (1,1′,2,2′,16,16′) of the sensor pair (1″,2″16″)are connected together antiparallel and are always operated alternatelywith respect to each other; c) on the basis of the polarity of thesignals, the switching amplifier (13) establishes the association of thesensor signals of a sensor pair (1″,2″16″) that are transmitted via thesame connection wires (5,5′,6,6′) to the sensor (1,1′,2,2′,16,16′) inquestion; d) the current source or voltage source (9) is a currentsource or voltage source with an alternating polarity.
 2. Sensor andevaluation system according to claim 1, characterized in that in orderto form the alternating current or alternating voltage interface (7,7′)of the connection wires (5,6) that establish the connection to theswitching amplifier, a short connection wire (5′,6′) with aseries-connected diode (8,8′,14,14′,17,17′) branches off in the vicinityof each of the sensors (1,1′,2,2′,16,16′), whereby each sensor(1,1′,2,2′,16,16′) of the sensor pair (1″,2″16″) is connected to twoconnection wires (5,5′,6,6′)—one of them with a diode(8,8′,14,14′,17,17′)—having opposite polarity, i.e. the sensors of asensor pair (1″,2″16″) are connected together antiparallel.
 3. Sensorand evaluation system according to claim 1, characterized in that thealternating current/voltage interface (7) is set up with reverse-connectprotection diodes (14,14′) that are already integrated into the sensor(1,1′,2,2′).
 4. Sensor and evaluation system according to claim 1,characterized in that the sensors (1, 1′,2,2′, 16,16′) are electronicsensors or mechanical contacts.
 5. Sensor and evaluation systemaccording to claim 1, characterized in that the connection wire (6)leading to the switching amplifier (13) is followed by at least onepolarized comparator as an evaluation unit (10,10′) with a storagedevice (11,11′) and said comparator generates two static signals at theoutput stage (12,12′), corresponding to the state of the sensors(1,1′,2,2′,16,16′) of the sensor pair (1′,2″16″).
 6. Sensor andevaluation system according to claim 1, characterized in that for eachsensor pair (1″,2″16″), the current is provided separately to thesensors (1,1′,2,2′) for an evaluation of the signal for line breaks andline short circuits.
 7. Sensor and evaluation system according to claim1, characterized in that the control circuit is galvanically separatedfrom the current source or voltage source (9) and from the evaluationunit (10,10′) and the subsequent elements (11,11′,12,12′).
 8. Sensor andevaluation system according to claim 1, characterized in that thesensors (1,1′,2,2′,16,16′) are polarized analog transmitters and theevaluation units (10,10′) are polarized analog inputs or A/D transducerswith storage behavior.
 9. Sensor and evaluation system according toclaim 1, characterized in that more than two sensors (19, 19′) areoperated via a shared control circuit (18), whereby there aresynchronized sampling units (20, 20′, 21, 21′) before the sensor and inthe switching amplifier, so that one sensor is switched on briefly at agiven point in time but never at the same time as another sensor, andthe analyzed sensor signal is associated with the appropriate outputstage.
 10. Sensor and evaluation system for acquiring data at ameasuring point (3), especially for double sensors for detecting finalpositions and limit values, consisting of at least two sensors(1,1′,2,2′,16,16′) that are connected via a control circuit to aremotely located switching amplifier (13) with a current source orvoltage source (9), whereby a connection wire (5,5′) leads from thecurrent source or voltage source to the sensor (1,1′,2,2′,16,16′) andanother connection wire (6,6′) leads from the sensor (1,1′,2,2′,16,16′)to an evaluation unit (10,10′), and the sensor signal of each sensor(1,1′,2,2′,16,16′) normally consists of a change in the current flow ascompared to the current flow in the quiescent state of the sensor(1,1′,2,2′,16,16′) and said sensor signal is transmitted via the controlcircuit to the switching amplifier (13) in order to be evaluated therein an evaluation unit (10,10′) and to be output at an output stage(12,12′) in the form of signals (e.g. binary ZERO, ONE) as a function ofthe sensor (1,1′,2,2′,16,16′) and of the time, characterized by thefollowing features: a) two sensors (1,1′,2,2′,16,16′)—a sensor pair(1″,2″16″)—are associated together with each of the two connection wires(5,5′,6,6′) to form the control circuit; b) an alternating current oralternating voltage interface (7,7′) is associated with each sensor pair(1″,2″16″), and said interface (7,7′) is located in the vicinity of thesensors (1,1′,2,2′,16,16′) and is capable of transmitting positivesignals to and from one sensor (1,2,16) as well as negative signals toand from the other sensor (1′,2′,16′) of the sensor pair (1″,2″16″),whereby the two sensors (1,1′,2,2′,16,16′) of the sensor pair (1″,2″16″)are connected together antiparallel and are always operated alternatelywith respect to each other; c) on the basis of the polarity of thesignals, the switching amplifier (13) establishes the association of thesensor signals of a sensor pair (1″,2″16″) that are transmitted via thesame connection wires (5,5′,6,6′) to the sensor (1,1′,2,2′,16,16′) inquestion; d) the current source or voltage source (9) is a currentsource or voltage source with an alternating polarity, wherein more thantwo sensors are operated via a shared control circuit, whereby there aresynchronized sampling units before the sensor and in the switchingamplifier, so that one sensor is switched on briefly at a given point intime but never at the same time as another sensor, and the analyzedsensor signal is associated with the appropriate output stage.
 11. Asensor and evaluation system for acquiring data at a measuring point (3)comprising a current source or voltage source (9); a remotely locatedswitching amplifier (13) connected to the current source or voltagesource (9); a first sensor (1,1′,2,2′,16,16′) connected through thecontrol circuit to the remotely located switching amplifier (13)connected to the current source or voltage source (9); a second sensor(1,1′,2,2′,16,16′) connected through the control circuit to the remotelylocated switching amplifier (13) connected to the current source orvoltage source (9); a first connection wire (5,5′) leading from thecurrent source or voltage source (9) to the first sensor(1,1′,2,2′,16,16′); an evaluation unit (10, 10′); an output stage(12,12′); a second connection wire (6,6′) leading from the second sensor(1,1′,2,2′,16,16′) to the evaluation unit (10,10′), wherein the firstsensor, the second sensor, the first connection wire and the secondconnection wire form a control circuit, and wherein a sensor signal ofthe second sensor (1,1′,2,2′,16,16′) normally consists of a change in acurrent flow as compared to a quiet current flow in a quiescent state ofthe sensor (1,1′,2,2′,16,16′) and wherein said sensor signal istransmitted via the control circuit to the switching amplifier (13) inorder to be evaluated in the evaluation unit (10,10′) and to be outputat an output stage (12,12′) in the form of signals (e.g. binary ZERO,ONE) as a function of the second sensor (1,1′,2,2′,16,16′) and of thetime, wherein the first sensor and the second sensor form a sensor pair(1″,2″16″) and are associated together with each of the first connectionwire and of the second connection wire (5,5′,6,6′) to form a controlcircuit; an alternating current or alternating voltage interface (7,7′)associated with the sensor pair (1″,2″16″), and wherein said interface(7,7′) is located in the vicinity of the first sensor and of the secondsensor (1,1′,2,2′,16,16′) and wherein said interface (7,7′) is capableof transmitting positive signals to and from the first sensor (1,2,16)as well as negative signals to and from the second sensor (1′,2′,16′) ofthe sensor pair (1″, 2″ 16″), whereby the first sensor and the secondsensor (1,1′,2,2′,16,16′) of the sensor pair (1″,2″16″) are connectedtogether antiparallel and are always operated alternately with respectto each other; wherein the switching amplifier (13) establishes theassociation of the sensor signals of a sensor pair (1″,2″16″) that aretransmitted via the same connection wires (5,5′,6,6′) to the sensor(1,1′,2,2′,16,16′) in question on the basis of the polarity of thesignals; wherein the current source or voltage source (9) is a currentsource or voltage source with an alternating polarity.